APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

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APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by APOD Robot » Wed Aug 10, 2022 4:05 am

Image Dust Clouds of the Pacman Nebula

Explanation: Stars can create huge and intricate dust sculptures from the dense and dark molecular clouds from which they are born. The tools the stars use to carve their detailed works are high energy light and fast stellar winds. The heat they generate evaporates the dark molecular dust as well as causing ambient hydrogen gas to disperse and glow red. Pictured here, a new open cluster of stars designated IC 1590 is nearing completion around the intricate interstellar dust structures in the emission nebula NGC 281, dubbed the Pac-man Nebula because of its overall shape. The dust cloud on the upper left is classified as a Bok Globule as it may gravitationally collapse and form a star -- or stars. The Pacman Nebula lies about 10,000 light years away toward the constellation of Cassiopeia.

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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by Ann » Wed Aug 10, 2022 10:12 am

Pacman Nebula Martin Rusterholz.png
Pacman Nebula by Martin Rusterholz.

Nebulas these days are almost always portrayed in narrowband images. Today's APOD was photographed through Hα (656 nm) and OIII (501 nm) filters. The red data through the Hα filter has been shown as red, of course, but the greenish cyan data from OIII has probably been mapped as blue (or bluish cyan).

I thought it might be fun to compare today's APOD with an RGB picture of the Pacman Nebula. As you can see, the nebula itself looks much darker in Martin Rusterholz's image than it does in the APOD, and the ionizing and illuminating star stands out in the RGB picture.
Wikipedia wrote about the ionizing star:

Multiple star 'B 1' or β 1 was later discovered by S. W. Burnham, whose bright component is identified as the highly luminous O6 spectral class star, HD 5005 or HIP 4121. It consists of an 8th-magnitude primary with four companions at distances between 1.4 and 15.7 arcsec.
The brightness of the star and the faintness of the nebula in the RGB image reminds me of what happened way back when, when the Hubble Space Telescope was brand new and not fully calibrated (I think), and some astronomers wanted to test Hubble by having it photograph a nebula in the Large Magellanic Cloud. (No, I can't believe it was the Tarantula Nebula.)

Anyway. The astronomers took the picture and looked at the result. To their horror, they couldn't see the nebula at all, just some bright stars! That was because those stars are so very, very much brighter than the nebula they illuminate. (There is a lot more to this story, I'm sure, but this is what I can remember from reading about it in Sky & Telescope in the 1990s.)

I guess the lesson is that if you want to photograph a nebula in RGB, you are going to have to do a lot of processing to bring out the nebulosity. But if you are photographing it through filters that isolate the specific wavelengths at which the nebula glows, the nebula will look quite bright. But the ionizing star(s) may be lost in the glare.

Ann
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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by orin stepanek » Wed Aug 10, 2022 12:09 pm

Pacman_Struble_1222.jpg
I don't see Pacman; but that's OK as I stil enjoy the nebula! 8-)
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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by acelepage » Wed Aug 10, 2022 12:46 pm

The whole image is Pacman. If you reduce the size of the image to a thumbnail, the likeness becomes more evident.

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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by orin stepanek » Wed Aug 10, 2022 1:07 pm

acelepage wrote: Wed Aug 10, 2022 12:46 pm The whole image is Pacman. If you reduce the size of the image to a thumbnail, the likeness becomes more evident.
Thanks! 8-)
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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by johnnydeep » Wed Aug 10, 2022 8:27 pm

Ann wrote: Wed Aug 10, 2022 10:12 am ...
The brightness of the star and the faintness of the nebula in the RGB image reminds me of what happened way back when, when the Hubble Space Telescope was brand new and not fully calibrated (I think), and some astronomers wanted to test Hubble by having it photograph a nebula in the Large Magellanic Cloud. (No, I can't believe it was the Tarantula Nebula.)
...
What do you mean by that parenthetical remark?
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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by Ann » Thu Aug 11, 2022 4:17 am

johnnydeep wrote: Wed Aug 10, 2022 8:27 pm
Ann wrote: Wed Aug 10, 2022 10:12 am ...
The brightness of the star and the faintness of the nebula in the RGB image reminds me of what happened way back when, when the Hubble Space Telescope was brand new and not fully calibrated (I think), and some astronomers wanted to test Hubble by having it photograph a nebula in the Large Magellanic Cloud. (No, I can't believe it was the Tarantula Nebula.)
...
What do you mean by that parenthetical remark?
I mean that the Tarantula Nebula is so bright that you can't miss it, even without processing.

Let's compare with the nebulosities in Orion:


As you can see, there are several nebulosities here. There is the Orion Nebula, of course, and the Running Man Nebula just above it, but there is also Barnard's Loop, and there is the large round Lambda Orionis nebula at top, and there is the red background to the Horsehead Nebula (even though the Horsehead itself is just barely visible as a tiny dark shape). You can see the small but semi-bright slightly yellowish Flame Nebula to the upper left of Alnitak, one of the Belt stars. And you can even faintly make out the Witch Head Nebula at lower right.

Here's my point: There is only one bright nebula here - well, two or possibly three, if you count the Running Man Nebula just above the Orion Nebula and the Flame Nebula to the upper left of Alnitak. In reality not even the Running Man Nebula or the Flame Nebula count: there is one bright nebula in Orion, and that is the Orion Nebula. You can't miss it. But you can miss all the others.


Let's look at the Large Magellanic Cloud with the help of an extremely cool gif showing SN 1987A blinking on and off:


Cool, eh? It gives you a real sense of what SN 1987 was like. The supernova was so obvious, and yet, for a supernova, it was kind of faint.

But here's the deal. You can see one obvious nebula in the gif, can't you? And that is the Tarantula Nebula.

You can't miss it. If NASA had tested the HST on the Tarantula Nebula, they couldn't have missed it.

Ann
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Re: APOD: Dust Clouds of the Pacman Nebula (2022 Aug 10)

Post by Chris Peterson » Thu Aug 11, 2022 1:18 pm

Ann wrote: Thu Aug 11, 2022 4:17 am You can't miss it. If NASA had tested the HST on the Tarantula Nebula, they couldn't have missed it.
Don't be so sure. It depends on the exposure time (which could have been very low if this was while they were still getting calibration data), and it also depends on contrast. The nebula fills a Hubble frame, so you don't really see it against a background. Although it has dust in it, it's still not a high contrast object. So it's easy enough for me to believe that in an initial test, that nebula might be quite unapparent in an image. Just bright stars against a low contrast background that would be hard to distinguish from any other background.
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